JP5716195B2 - Hydrogen storage alloy and alkaline storage battery using this hydrogen storage alloy - Google Patents

Description

  The present invention relates to a hydrogen storage alloy and an alkaline storage battery using the hydrogen storage alloy, and more particularly to a hydrogen storage alloy that contributes to improving the cycle life characteristics of the alkaline storage battery.

  Nickel metal hydride storage batteries are known as one of alkaline storage batteries. This nickel metal hydride storage battery has a high capacity and excellent environmental safety compared to nickel cadmium storage batteries. It is used for various applications such as hybrid electric vehicles.

Examples of the hydrogen storage alloy used for the negative electrode of the nickel-metal hydride storage battery include a LaNi ₅ -based hydrogen storage alloy that is a rare earth-Ni-based hydrogen storage alloy having an AB ₅ type structure with a CaCu ₅ type crystal as a main phase, and Ti In general, a hydrogen storage alloy having an A ₂ B ₄ type structure having a Laves phase crystal containing Zr, V, and Ni as a main phase is used.
However, the hydrogen storage alloy as described above does not necessarily have sufficient hydrogen storage capacity, and it has been difficult to increase the capacity in the case of a nickel-metal hydride storage battery using this hydrogen storage alloy.

Therefore, in recent years, in order to improve the hydrogen storage capacity of the hydrogen storage alloy, a rare earth-Mg-Ni hydrogen storage alloy having a composition in which a part of the rare earth element of the rare earth-Ni system hydrogen storage alloy is replaced with Mg has been proposed. Has been. This rare earth-Mg—Ni-based hydrogen storage alloy has a structure including an AB ₅ type subunit and an A ₂ B ₄ type subunit, and the storage and release of hydrogen, which is a feature of the AB ₅ type alloy, is stable. And the advantage of a large amount of hydrogen storage, which is a feature of the A ₂ B ₄ type alloy. Therefore, it has been reported that the rare earth-Mg-Ni-based hydrogen storage alloy stores a larger amount of hydrogen gas than conventional rare-earth-Ni-based hydrogen storage alloys (see Non-Patent Document 1).

Journal of Alloys and Compounds, Volume 311, Issue 2, p. L5-L7 (2000) (Journal of Alloys and Compounds Volume 311, Issue 2, Pages L5-L7 (2000))

  A battery using the rare earth-Mg-Ni-based hydrogen storage alloy as a negative electrode material can increase the capacity, but has the following problems.

The A ₂ B ₄ type subunit contained in the rare earth-Mg—Ni-based hydrogen storage alloy is not easily cracked due to the expansion of the crystal lattice during the storage of hydrogen, but the crystal structure is likely to be distorted, If the residual amount of such strain increases, there arises a problem that the crystal structure itself becomes a form in which hydrogen cannot be occluded and released. That is, the A ₂ B ₄ type subunit has a large amount of hydrogen occlusion, but causes degradation due to distortion of the crystal structure as it repeatedly occludes and releases hydrogen. For this reason, if the battery is repeatedly charged and discharged, the amount of occlusion and release of hydrogen decreases due to the deterioration due to the distortion of the crystal structure in the A ₂ B ₄ type subunit, and the discharge capacity decreases accordingly.

On the other hand, in the AB type ₅ subunit, the alloy is easily cracked due to the expansion of the crystal lattice during the occlusion of hydrogen, and the alloy breaks and becomes fine powder when repeated occlusion and release of hydrogen. In this way, as the alloy breaks, many new surfaces with high reactivity are generated, so the electrolyte in the battery reacts with the new surface, and the alloy is oxidized and deteriorates, reducing the amount of hydrogen stored. As a result, the discharge capacity is reduced. Moreover, in the reaction between the electrolyte and the new surface, the electrolyte is consumed and reduced, and as a result, the internal resistance of the battery increases and discharge becomes difficult. Therefore, the discharge capacity decreases due to the consumption of the electrolyte. Also happens.

  Thus, in the alkaline storage battery using the rare earth-Mg-Ni-based hydrogen storage alloy as described above as the negative electrode material, repeated charge / discharge results in a decrease in discharge capacity, resulting in a cycle life of the battery. There was a problem of shortening.

  The present invention has been made to solve the above-mentioned problems in alkaline storage batteries having a rare earth-Mg-Ni-based hydrogen storage alloy in the negative electrode. The object of the present invention is to repeatedly charge and discharge the battery. In some cases, it is an object to provide a hydrogen storage alloy that can maintain a high capacity for a long period of time and contribute to an improvement in the cycle life characteristics of the battery, and an alkaline storage battery using the hydrogen storage alloy.

In order to achieve the above object, according to the present invention, a rare-earth-Mg-Ni-based hydrogen storage alloy comprising a mixed phase composed of A ₂ B ₄ type subunits and AB ₅ type subunits, The mixed phase includes a basic unit represented by [LHLHHH] (where L represents an A ₂ B ₄ type subunit, H represents an AB ₅ type subunit), and this basic unit has a crystal axis in a crystal structure. Among these, a hydrogen storage alloy characterized by being laminated in a uniaxial direction is provided.

  Moreover, according to this invention, the alkaline storage battery provided with the negative electrode element containing the hydrogen storage alloy of Claim 1 is provided (Claim 2).

The hydrogen storage alloy according to the present invention includes a mixed phase composed of an A ₂ B ₄ type subunit and an AB ₅ type subunit, and the mixed phase is [LHLHHH] (where L is an A ₂ B ₄ type subunit). Unit, H represents an AB type ₅ subunit), and has a novel structure in which a basic unit is stacked. With such a structure, hydrogen is occluded and released while maintaining the amount of hydrogen occluded. Deterioration of the alloy due to is suppressed. Moreover, the alkaline storage battery of this invention provided with the negative electrode element containing this hydrogen storage alloy becomes the thing excellent in cycle life characteristics.

It is the model figure which showed typically the basic unit of the crystal structure in the hydrogen storage alloy which concerns on this invention. It is the schematic diagram which showed schematic structure of the open type nickel metal hydride storage battery. 2 is a high-angle scattering annular dark field scanning transmission microscope (HAADF-STEM) photograph showing a lattice image of the hydrogen storage alloy according to Example 1. FIG. 6 is a model diagram schematically showing a basic unit of a crystal structure in a hydrogen storage alloy according to Comparative Example 1. FIG. It is the graph which showed the relationship between the cycle number and a capacity | capacitance maintenance factor.

A hydrogen storage alloy according to an embodiment of the present invention is a rare earth-Mg—Ni-based hydrogen storage alloy containing a rare earth element, magnesium, nickel, aluminum, or the like at a predetermined ratio, and is a sub-type of A ₂ B ₄ type. It contains a mixed phase consisting of units and AB ₅ type subunits.

In this mixed phase, a large number of basic units 2 shown in FIG. 1 are periodically stacked, for example, in the direction indicated by the arrow in FIG. 1, that is, in the direction of the so-called c-axis, which is the main axis among the crystal axes in the crystal structure. It has the structure which becomes. Here, as is apparent from FIG. 1, the basic unit 2 includes an A ₂ B ₄ type subunit represented by a reference symbol L and an AB ₅ type subunit represented by a reference symbol H at a specific period c. It is laminated in the axial direction. Specifically, the layers are stacked in the order of LHLHHH from the top to the bottom in FIG. That is, the mixed phase has a periodicity of [LHLHHH] n (where L is an A ₂ B ₄ type subunit, H is an AB ₅ type subunit, and n is an integer).

  Since the hydrogen storage alloy according to the present invention includes the mixed phase as described above, the deterioration of the alloy is suppressed even when the storage and release of hydrogen is repeated. As a result, the alkaline storage battery using the hydrogen storage alloy according to the present invention as the negative electrode material has improved cycle life characteristics.

  Here, it is considered that the alkaline storage battery using the rare earth-Mg-Ni-based hydrogen storage alloy according to the present invention as the negative electrode material has excellent cycle life characteristics for the following reason.

The basic unit 2 in the rare earth-Mg-Ni-based hydrogen storage alloy according to the present invention can be divided into two regions for convenience. That is, the basic unit 2, as shown in FIG. _1, a first region 4 A 2 _{B 4} type subunit and AB ₅ type subunits are arranged in a one-to-one _ratio, A 2 _{B 4} type subunit And AB type ₅ subunits can be divided into a second region 6 arranged in a ratio of 1: 3. The first region 4 is positioned on the upper side in FIG. Are connected. Here, the first region (A ₂ B ₄ : AB ₅ = 1: 1) 4 has a so-called AB ₃ type structure, and the second region (A ₂ B ₄ : AB ₅ = 1: 3) 6 has a so-called structure. A ₅ B ₁₉ type structure. As the hydrogen is occluded and released, the AB ₃ type structure is not easily broken but easily distorted, and the A ₅ B ₁₉ type structure is easily distorted but easily broken. The characteristics of the AB ₃ type structure and the A ₅ B ₁₉ type structure are determined by the composition ratio of the A ₂ B ₄ type subunit and the AB ₅ type subunit contained therein. That is, in the AB ₃ type structure, the crystal structure is easily distorted (difficult to break) due to the A ₂ B ₄ type subunit, and in the A ₅ B ₁₉ type structure, the alloy is caused by the AB ₅ type subunit. The ease of cracking (hardness of distortion) is manifested.

The mixed phase contained in the hydrogen storage alloy of the present invention has a structure in which the basic unit 2 described above repeats periodically, but when viewed in more detail, A ₂ B ₄ : AB corresponding to the AB ₃ type structure. The first region of ₅ = 1: 1 and the second region of A ₂ B ₄ : AB ₅ = 1: 3 corresponding to the A ₅ B ₁₉ type structure are periodically repeated. Because of such a structure, the second region (A ₅ B ₁₉ type structure) absorbs the influence of strain generated in the first region (AB ₃ type structure) due to the storage and release of hydrogen, and the second region (A ₅ B ₁₉ type structure) It is considered that the influence of the ease of cracking (A ₅ B ₁₉ type structure) can be absorbed in the first region (AB ₃ type structure). Therefore, it is considered that the hydrogen storage alloy as a whole can suppress the distortion caused by the storage and release of hydrogen and the deterioration due to pulverization. Thus, if distortion and pulverization of the hydrogen storage alloy are suppressed, the AB ₅ type subunit and the A ₂ B ₄ type subunit inherently excellent hydrogen storage / release capability can be maintained for a long period of time. Therefore, it is considered that the alkaline storage battery using the hydrogen storage alloy of the present invention as the negative electrode material can maintain a high capacity for a long period of time even after repeated charge and discharge, and has excellent cycle life characteristics.

Next, the hydrogen storage alloy of this invention is obtained as follows, for example.
First, metal raw materials are weighed and mixed so as to have a predetermined composition, and this mixture is melted in, for example, a high-frequency induction melting furnace and then cooled to an ingot. The obtained ingot is heated to 900 to 1200 ° C. in an inert gas atmosphere and subjected to a heat treatment for 24 to 78 hours to obtain the hydrogen storage alloy of the present invention. Thereafter, the ingot is pulverized and classified to a desired particle size by sieving to obtain hydrogen storage alloy particles.

Here, the composition of the rare earth-Mg—Ni-based hydrogen storage alloy according to the present invention can be freely selected.
Ln _1-x Mg _x Ni _y-a-b Al _a M _b (I)
It is preferable to use what is represented by these.

  However, in the general formula (I), Ln is derived from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y, Zr and Ti. Represents at least one element selected from the group consisting of M, V, Nb, Ta, Cr, Mo, Mn, Fe, Co, Ga, Zn, Sn, In, Cu, Si, P, and B. Represents at least one element selected from the group, and the subscripts a, b, x, and y are 0.05 ≦ a ≦ 0.30, 0 ≦ b ≦ 0.50, and 0.05 ≦ x ≦ 0. 30 represents a number satisfying 2.8 ≦ y ≦ 3.9.

1. Example 1
(1) Production of hydrogen storage alloy electrode (negative electrode) Nd, Mg, Ni and Al are weighed and mixed so as to have a predetermined composition, and the resulting mixture is melted in a high frequency induction melting furnace, and the molten metal is used as a mold. And cooled to room temperature to obtain a hydrogen storage alloy ingot. In addition, as a result of analyzing the composition of this hydrogen storage alloy by high frequency plasma spectroscopy (ICP), the composition was Nd _0.75 Mg _0.25 Ni _3.35 Al _0.15 .
The hydrogen storage alloy ingot was then heated to 950 ° C. in an argon gas atmosphere and maintained for 48 hours.

Next, the heat-treated ingot was mechanically pulverized in an argon gas atmosphere to obtain a rare earth-Mg—Ni-based hydrogen storage alloy powder having an average particle size of 65 μm.
1 part by weight of the obtained hydrogen storage alloy powder and 3 parts by weight of nickel powder having an average particle diameter of 2.5 μm as a conductive agent are mixed, and the resulting mixture is pressure-molded to give 1 g of a pellet-shaped hydrogen storage alloy negative electrode 14 was produced.

(2) Production of Nickel Metal Hydride Battery An open type liquid rich nickel metal hydride storage battery 10 shown in FIG. 2 was produced. The nickel metal hydride storage battery 10 includes a polypropylene container 12, and the negative electrode 14 of the hydrogen storage alloy obtained as described above in the container 12 and a negative electrode capacity regulation at a position facing the negative electrode 14. A reference electrode 16 made of a mercury oxide electrode was disposed. Furthermore, a cylindrical positive electrode 18 was disposed so as to surround the negative electrode 14 and the reference electrode 16. The positive electrode 18 is made of sintered nickel having a sufficiently large capacity with respect to the negative electrode 14. Then, an alkaline electrolyte 20 made of a 7N KOH solution was poured into the container 12 so that the negative electrode 14, the reference electrode 16, and the positive electrode 18 were completely immersed, and an open-type nickel-metal hydride storage battery 10 was produced. In FIG. 2, reference numerals 22, 24, and 26 represent leads connected to the negative electrode 14, the reference electrode 16, and the positive electrode 18, respectively. These leads 22, 24, and 26 are a charging power source and a measuring instrument (not shown). Etc. are connected.

2. Comparative Example 1
An ingot of a hydrogen storage alloy having a composition of (La _0.20 Pr _0.4 Nd _0.4 ) Mg _0.17 Ni _3.13 Al _0.17 was prepared, and this ingot was heated at 1000 ° C. in an argon gas atmosphere at 10 ° C. A hydrogen storage alloy negative electrode similar to that in Example 1 was prepared except that the heat treatment was performed for a time. And the open-type nickel metal hydride storage battery 10 was produced like Example 1 except having used the negative electrode obtained in this way.

3. Evaluation of hydrogen storage alloy (1) Observation of high-angle scattering annular dark field scanning transmission microscope (HAADF-STEM) A sample for HAADF-STEM observation was previously collected from a hydrogen storage alloy ingot after heat treatment, and HAADF was collected from this sample. -STEM observation was performed.

(2) About Example 1 A HAADF-STEM photograph of the hydrogen storage alloy of Example 1 is shown in FIG.
First, in the HAADF-STEM photograph of FIG. 3, an atomic column consisting only of a rare earth element having a large atomic number contained in the AB type ₅ subunit is indicated by a bright bright spot, so that a low magnification image (FIG. 3A) in, can be bright line perpendicular to the c-axis direction is not at regular intervals, to confirm the repetition of a thick bright lines caused by the AB ₅ type subunit 3 AB ₅ type subunits one thin bright line.

Moreover, in FIG.3 (b) which expanded a part of Fig.3 (a), the model figure for demonstrating the observed crystal structure was superimposed and displayed. As is clear from FIG. 3 (b), the hydrogen storage alloy according to Example 1 has an A ₂ B ₄ type subunit (L) and an AB ₅ type subunit (H) represented by LH of 1: 1. And A ₂ B type ₄ subunits (L) and AB type ₅ subunits (H) are alternately stacked in a region represented by 1: 3 LHHH, and the basic unit represented by LHLHHH as a whole It can be seen that the structure includes a periodically laminated structure. That is, also from the HAADF-STEM photograph of the hydrogen storage alloy of Example 1, the hydrogen storage alloy of the present invention has a structure in which the basic unit 2 having the LHLHHH structure shown in FIG. 1 is periodically stacked in the c-axis direction. It can be seen that

(3) About Comparative Example 1 On the other hand, as for the hydrogen storage alloy of Comparative Example 1, from the observation result of the HAADF-STEM photograph, the period of the bright line is equal, and the A ₂ B ₄ type subunit (L) and AB It was confirmed that the basic unit represented by LHH of 1: 2 type ₅ subunit (H) was repeatedly laminated. A model diagram of the basic unit 32 of Comparative Example 1 obtained based on the observation result of the HAADF-STEM photograph is shown in FIG.

(4) From these FIG. 3 (b) and FIG. 4, in the hydrogen storage alloy of Example 1 and the hydrogen storage alloy of Comparative Example 1, the A ₂ B ₄ type subunit (L) and the AB ₅ type subunit (H) It can be seen that there is a difference in the crystal structure and the crystal structure.

4). Evaluation of Nickel Metal Hydride Battery (1) The obtained open nickel metal hydride battery 10 is charged at a temperature of 25 ° C. for 170 minutes with a current of 300 mA with respect to 1 g of the hydrogen storage alloy, and then charged after resting for 10 minutes. The operation was repeated 50 times with the same current as the time being discharged until the voltage of the negative electrode 14 with respect to the reference electrode 16 (mercury oxide electrode) became −0.7 V, and then resting for 10 minutes as one cycle. And the discharge capacity | capacitance for every cycle was measured, and the measured value was made into cycle capacity. Further, the maximum value among the cycle capacities was set as the maximum capacity. And the capacity | capacitance maintenance factor with respect to the maximum capacity | capacitance shown by (II) Formula was calculated | required.
Capacity maintenance ratio with respect to the maximum capacity (%) = (cycle capacity / maximum capacity) × 100 (II)

  The relationship between the number of cycles and the capacity retention rate is shown in FIG. In addition, the capacity retention ratio with respect to the maximum capacity at the time of final charge / discharge was 98.9% in Example 1 and 93.0% in Comparative Example 1.

(2) From FIG. 5, the following is clear.
(I) The battery using the hydrogen storage alloy of Example 1 takes a relatively long time to activate, but after exhibiting the maximum capacity value, the capacity retention rate decreases little. The capacity retention rate is as high as 9%.
This is presumably because the hydrogen storage alloy of Example 1 hardly deteriorates with the storage and release of hydrogen, and the decrease in the amount of storage and release of hydrogen could be suppressed, so that the battery capacity could be maintained relatively high.

The structure of the hydrogen storage alloy of Example 1 is that the A ₂ B ₄ type subunit (L) and the AB ₅ type subunit (H) are represented by a first region represented by 1: 1 LH, and A ₂ B ₄ Type subunits (L) and AB ₅ type subunits (H) are alternately stacked in a second region represented by 1: 3 LHHH, and a basic unit represented by LHLHHH as a whole is periodically laminated. Therefore, the first region having the characteristics of being hard to break but easily distorted and the second region having the characteristics of being hard to crack but easily fragile complement each other, and as a whole, crystals accompanying the absorption and release of hydrogen This is considered to be because the occurrence of structural distortion and alloy cracking can be effectively suppressed.

(Ii) The battery using the hydrogen storage alloy of Comparative Example 1 completed activation at an early stage, and exhibited a maximum capacity value before 10 cycles. After that, the capacity retention rate has decreased and finally has decreased to 93.0%.
This is presumably because the hydrogen storage alloy of Comparative Example 1 easily deteriorates due to the storage and release of hydrogen, leading to a decrease in the amount of storage and release of hydrogen, so that the capacity of the battery was reduced early.

The structure of the hydrogen storage alloy of Comparative Example 1 is a structure in which A ₂ B ₄ type subunit (L) and AB ₅ type subunit (H) are repeatedly laminated with basic units represented by LHH of 1: 2. Therefore, the ease of cracking of the alloy due to the AB type ₅ subunit cannot be sufficiently supplemented by the portion of the type A ₂ B ₄ type subunit having the characteristics that the crystal structure is easily distorted and difficult to break. This is probably because the deterioration has progressed.

(3) From the above, the nickel-metal hydride secondary battery using the hydrogen storage alloy of the present invention as the negative electrode material can suppress a decrease in the hydrogen storage amount due to repeated charge and discharge, and can maintain a high capacity over a long period. It can be said that it has excellent life characteristics.

  In addition, this invention is not limited to an above-described Example, A various deformation | transformation is possible. For example, the alkaline storage battery using the negative electrode element containing the hydrogen storage alloy according to the present invention is not limited to the open nickel-metal hydride storage battery described above, and may be used for a sealed nickel-metal hydride storage battery.

2 Basic unit 4 1st area 6 2nd area 10 Open type nickel metal hydride storage battery 12 Container 14 Negative electrode 16 Reference electrode 18 Positive electrode 20 Alkaline electrolyte 22, 24, 26 Lead L A ₂ B ₄ type subunit H AB ₅ type sub unit

Claims (2)

A rare earth-Mg-Ni based hydrogen storage alloy,
Including a mixed phase composed of A ₂ B type ₄ subunit and AB type ₅ subunit,
The mixed phase is
[LHLHHH] (where L is an A ₂ B ₄ type subunit, H is an AB ₅ type subunit), and this basic unit is laminated in the uniaxial direction among the crystal axes in the crystal structure. Hydrogen storage alloy characterized by being made.   An alkaline storage battery comprising a negative electrode element comprising the hydrogen storage alloy according to claim 1.